The simple explanation is that draft picks must either be offered NBA contracts or released. If the players choose to sign elsewhere (often at the team's urging, like Ante Zizic and Guerschon Yabusele did), as Nader will do with Maine, the club can then retain their draft rights. However, if they instead opt for an NBA deal and don't make the team (Ben Bentil is a likely example of this), they become free agents.

By inking a D-League contract, Nader will now remain under the Celtics' control. He may also be betting on someday making the team. Chances are Nader could've earned significantly more money by going to camp, getting cut, then signing elsewhere (or, as Shams Charania's tweet mentions, going to Europe).